Trash and debris floating on the surfaces of waterways or along shorelines and beaches is a highly visible form of water pollution, which is receiving attention for its adverse, polluting effect and for its unaesthetic appearance on the surfaces of lakes and other water bodies. One type of system for the collecting and removing of floating debris has consisted of arrays of disposable mesh nets installed in receiving bodies of water in the flow path of a sewer outlet, particularly in applications referred to as “Combined Sewer Overflows” or “CSOs”. Such systems are described in Vol. 2, No. 3, of Fresh Creek Technologies, Inc. “Shorelines” newsletter. Systems of this type are effective in collecting floatables or trash for removal and are shown in Fresh Creek Technologies, Inc. Netting Trashtrap™ Product Bulletin. Improvements in such devices are described in U.S. Pat. No. 5,562,819, owned by the assignee of the present application, which provides an underground, in-line apparatus for trapping and collecting debris in a sewer or storm flow conduit, a secondary trap which provides continued protection when primary collection traps are full, a system which signals when primary bags or nets are full and servicing is required, and a trapping facility in which bags or nets may be replaced without loss of trapping protection during servicing.
More specifically, the device in the patent referred to above includes an enclosure or chamber with an inlet and an outlet each adapted to be connected to a sewer, storm drain conduit or outflow. A debris removing system is disposed within the chamber between the inlet and the outlet for trapping and collecting water borne debris entering at the inlet and thereby providing for an outflow of substantially debris-free water. The enclosure includes an access opening comprising upper doors or hatches or access hatches in the enclosure sized to allow the debris removing system to be removed and replaced. The debris removing system specifically includes a perforated container having an open end facing the inlet of the chamber. The perforated container includes a netting assembly that traps and collects the trash or floating debris. The container is in the form of a netting assembly having a flexible bag-shaped mesh net attached to a frame. The netting assembly is attached to lifting structure having supports or handles for allowing the frame and net to be lifted out when the net is full of captured debris. In some applications, a bypass weir or screen is provided to normally direct flow from the chamber inlet through the open end of the net while allowing flow to bypass the net and flow to the chamber outlet when the net is full of debris.
Sensing and signaling elements are typically provided for sensing and signaling the passage of solid debris around the net when the net is full of debris and is in need of servicing. The sensing and signaling elements may include mechanical structure which permits passage of water, but is displaced by impingement of solid debris flowing around the nets. Displacement of such mechanical structure signals when the net is full of debris, for example, by actuating a visible flag above ground or by actuating an electrical switch which activates an aboveground indicator or remote indicator. The sensing and signaling may include an optical sensor for detecting the passage of debris around the netting assembly. Upon detection of debris, the optical sensor emits a signal indicating that the trap is full of debris. The signal may also activate an aboveground indicator or a remote indicator.
Multiple trap systems are employed in which the enclosure includes side-by-side trap assemblies. Such systems maybe configured such that, upon filling of the first trap, the flow and debris can be diverted over a bypass weir disposed between the inlet ends of the first and second traps so that flow is thereby directed through the second trap and overflow debris is trapped and collected. Closure panels may be provided in a stationary frame structure disposed adjacent the inlet ends of the traps in either the single-trap systems or the multitrap systems to restrain debris from flowing through the chamber during servicing.
The reliability of debris removing systems depends on the strength of the mesh nets and on the manner in which the net material is fabricated into the disposable net assemblies. The resultant hydraulic forces are a function of the velocity of the flow of water through the mesh of the nets as well as on the pressure exerted on the debris trapped by the nets. There are many outfalls where extreme forces exist that are too high for standard and commonly available materials or for materials made by normal fabrication practices to last.
Furthermore, the operation of such debris removal systems results in the nets filling with floatable materials over time as one or more overflows occur. In the process, large objects such as plastic bottles and sheets of plastic wrapping materials tend to cover and blind openings of the mesh, which reduces the overall effective area of the filter and results in higher hydraulic loading on the mesh, contributing to a higher pressure drop through the system and increased loads, and excessive forces on the nets.
Accordingly, a need exists for stronger and more reliable mesh nets in the traps of floatable debris collecting systems, and particularly for net assemblies that can be easily constructed and easily replaced.